Hydraulic interferometer



April 8, 1947. F. H. NADlG ErAL l HYDRAULIC INTSRFEROMETER 2 Sheets-Shut Il,

Filed Oct. 16, 1943 Patented Apr. a", 1947 y Francis H.

: tal-18,786

Nadig, Philadelphia, and Jacob L. f Bohn, Glenside, Pa. y

' Applicatiooctoberis,1943. serial No. 506,514

-s claims. erase-1a 'I'his invention relates to an instrument for measuring light waves.. variance in 'length of certain substances and earth tides, manifested in Voptical interference.

More particularly, the invention relates to improvements in instruments, such as the Michelson interferometer.

An object of the invention is the provision of improved adjusting `means in an instrumentcf the indicated character, whereby to measure more minute' magnitudes with hitherto.

The nature of the invention and its distinguishing features. and advantages will appear when the following specification is read in connection with the accompanying drawings, in which- Fig. l is a side view of an instrument embodygreater accuracy than v in a cylinder i8.' Aconnectingconduit i9 con nects the lower end of' the cylinder I3 with a cylinder 26. An indicator plunger or piston 2i.' operates in the cylinder 20. 'The piston 2l isin the nature of apiece ofA wire. It has guidedmove-n ment in a g'landr b ushing Z A quantity of motion transmission liquid, such as mercury 23, iills thev cylinders and the conduit connecting the same. The cylinder 'Zliextends below the conduit .I9 in order to allow-.the wire 2i to have considerable movement in displacing the mercuryto compel the upward movement ofthe 'piston il and the mirror I 3 carried thereby.

distance between them. `The cylinders and transverse conduitconnectingthe same are formed :n

the base I0, but obviously these features vrnay be incorporatedin the instrument in any other-prac- I vtical manner for the intended purpose.

interferometer'to which the present invention is applied by way of example. The instrument includes a base Il) on which are mounted the. ad` Y justable frames which support the reilec-tors'consisting of the. so-called""half-silvered mirror II,

a vertical mirror I2, and a horizontal mirror I3.v

Attached to the base In is a' bracket I4 which.

supports a neon lamp I5 which sends a light beam to the mirror II.' This incident'light beam is divided by the mirror II intotwo rays'which byA virtue of the mirrors I2 and I3 are reversed so that there is a half a wave length of optical path differencebetween them, thereby producing the interference fringes or bands which may be seen in the telescope I6.

In accordance with the invention, hydraulic means is provided to move or adjust the mirror The movement of thev mirror I3 with respect to the mirror I-I in one direction causes the interel ference fringes to contract or converge toward a center point, and the fringes expand or diverge from the center point when the mirror I 3 is moved I in the opposite direction. The fringes are4 counted asv they move in measuring the length of a light wave as one of theuses of the instrument. The f provision and arrangement of themeans'set forth make it'possible to measure more minute magnitudes with greater accuracy'than hitherto. This is true because of the greatdisparity in the respective displacement characteristics of the4 piston Il and the plunger 2 I. .A greatmovement of the plunger 2l causes a relatively innitesmally small movement of the piston Il'. f By using a suitable be readily ascertained.

The instrumentcan be used in several ways. 'I I .Knowing the wave length of the light and count' lng the fringes the distance can be calculated.

Knowing the. distance mirror. I3 is moved andv I3 with respect to the mirror II, so that the surface of the former is always parallel t'o its original position. while its distance from themirror II is altered, to cause the fringes or bands to .shift across the ileld, to be counted as they move for a measuring operation. Said hydraulic means includes-a piston il on the upper end of which the mirror I3-is mounted. i The piston I1 is'movable counting the number of fringes, the wave length of the light can be calculated.

4The following isa specific example ofthe use of the instrument todetermine wave length of light.

Asthe plunger 2l' is moved4v downwardly. it

- displaces some mercury, which in turn tends to go into Cylinden I8,I thus forcing the piston Il andthe mirror I3 carried thereby, upwardly. In

the reverse operation, that is to say, when the plunger 2l is moved upwardly, the cylinder I8 By withdrawing the wire plunger ZI-,jthe mercuryrec'edes, thereby allowing the piston Il andthe' mirror I3 tomove downwardly. The mirrorv I 3, therefore, is movable toward and away from the mirror Il to alter the scale, the distance the -plunger 2i ismoved may A loses some mercury and the piston l1 and its mirror I3 move downwardly under the action of gravity.

It the diameter of plunger 2| is .032 centimeter, then the radius of plunger 2| is .016 cm.=rz. If the diameter of the piston l1 is 1.60 cms. then the radius of the piston l1 is .80 cm.=r1. Ii the plunger 2l is moved downwardiy 2.54 ems. then nomine equation Thus the wave length of this particular light is .0000546 centimeter. Thus the hydraulic lever principle in an interferometer is used to reduce the applied motion in measuring the wave length of light. The motion is reduced to of the applied motion.

If a plunger 2| o f a diameter larger than that of the piston |1 is used, a downward movement of the plunger 2| will move the piston l1 upwardly a distance greater than thevmovement of the plunger 2| depending upon the respective diameters oi said plunger and piston.

Suppose in the above case use is made of lightl .0000215 inch in wave length. To get 1 fringe shift, the mirror I3 must be moved .0000107 inch. This means that if a motion of .000000107 inch is applied to plunger 2| a 1 fringe shift will be obtained if the ratio of the diameters of the plunger and piston is 100 to 1. If a 1/10 of a fringe shift is observed, plunger 2i will have to be moved only .0000000107 inch. Thus a motion of approximately 1/100,000,000 of an inch may be detected. Inthis manner the instrument also may be used to detect small increments of length, or small decreases of length.

In Fig. the interferometer is shown for measuringearth tides. In this case the hydraulic lever is used to amplify the motion of a' plunger or piston |2| of relatively large diameter compared to that ci the piston l1 which carries the mirror I3. In order to"4 measure earth tides the variation of the acceleration due to gravity isascertained. This is accomplished by attaching a weight 25 to the lower end of a coil spring 26 whose upper end is fixed as at 21. The weight V25 is connected with the piston |2| by a bar 28. Variations in the length of the spring 2B aiect the hydraulic lever and such variation can be detected by the manifested optical interference.

To measure small increments of length, the inf. strument is in the form shown in Fig. 5 with the weight 25, spring 26 and bar 28 omitted. As

shown by way of example in -Fig. 6, a brass rod 29 is rigidly supported vertically as at 30. A second brass rod 3| Vrests vertically on the piston 12|.v An iron rod 32 is arranged between the rods 29 and 3| with its opposite ends in contact with the lower and upper ends respectively of the rods 29 and 3|. A coil 33 surrounds the rod A32.v When an electric current is passed through the coil 33, the iron rod 32 changes its length, causing the piston 12| to move accordingly. This motion may be observed and measured by the fringe shifts of the optical system of the instrument. Other small displacements may be applied to the piston |2| and be observed and measured by the shift of the fringes. By other Asmall displacements is meant changesl in the thickness of certain crystals due to the piezoelectric effect, changes iin the length of certain metal rods due to the phenomenon of magnetostriction, and changes in the dimensions vof materials due to small temperature variations.

By employing the hydraulic lever principle to move or adust one of the mirrors, a precision type of instrument may be produced at low cost, and one easy to maintain, particularly the type suitable for students.

We claim:

l. An interferometer comprising a light source. an eye-piece and means for reflecting the light from said source to said eye-piece in the form of interfering light beams, said reflecting means including a vertically movable horizontal mirror, a hydraulic system comprising connected cylinders of substantially different cross section, a piston in one cylinder carrying said movable mirror and a plunger` adjustable in the other cylinder for effecting the, movement of the piston.

2. An interferometer comprising a light source, an eye-piece and means for reflecting the light from said source to said eye-piece in the form of interfering light beams, said reflecting means including a vertically movable horizontal mirror, a hydraulic system comprising two connected cylinders of substantially different cross section, the smaller cylinder being of greater length, a piston in .the larger cylinder carrying said movable mirror, and a plunger in the smaller cylinder the adjustment of which through a wide range of movement eiects movement of the piston through a proportionately lesser range.

3. An interferometer comprising a light sourceV an eye-piece and means for reflecting the light from said source to said eye-piece in the form ofinterfering light beams, said reflecting means including a -vertically movable horizontal mirror, a hydraulic system comprising two cylinders of substantially different cross section and length and a connecting passage communicating with the smaller cylinder at a mid point, apiston in the larger cylinder carrying said movable mirror and a plunger movable longitudinally in the liquid column of the smaller cylinder serving to displace small quantities thereof by a relatively large movement of the plunger and eifect a proportionally smaller movement of the piston in thel larger cylinder.

4. An interferometer comprising a light source, an eye-piece and means for reflecting the light from said source to said eye-piece in the form of interfering light beams, said reflecting means 5 able longitudinally in the smaller column through a wide range oi' movement and serving to displace the mercury in the larger column to eilectproportionately small movements of the piston.

5. An interferometer comprising a light source, 5

an eye-piece and means for reflecting the light -:from said source to said eye-piece in the form oi EEFEnENcEs CITED The following references are of record in the iile of this patent:

l UNITED STATES PATENTS Number Name Date 1,044,502 Crehore et al Nov. 19, 1912 FOREIGN PATENTS Number Country Date 389,417 France June 27, 1908 486,694

Britain .Q June 9, 1938 

